R Coronae Borealis and dustless hydrogen-deficient carbon stars likely have different oxygen isotope ratios

Karambelkar, Viraj; Kasliwal, M. M.; Tisserand, Patrick; Clayton, Geoffrey C.; Crawford, Courtney L.; Anand, S.; Geballe, T. R.; Montiel, Edward

doi:10.1051/0004-6361/202142918

Cited by 11 publications

(10 citation statements)

References 41 publications

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“…Furthermore, the inferred number of galactic RCB stars (e.g., Tisserand et al 2020) is broadly consistent with that expected from the He WD-CO WD merger rate (e.g., Ruiter et al 2009;Karakas et al 2015), whereas the final flash stars spend too little time in the RCB phase to explain the number of known RCB stars (Clayton et al 2006). It has been suggested that dLHdC stars are similarly formed through mergers but with different mass ratios (e.g., Karambelkar et al 2022;Tisserand et al 2022).…”

Section: Introductionsupporting

confidence: 74%

The Asteroseismological Richness of RCB and dLHdC Stars

Wong,

Bildsten

2024

ApJ

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RCB stars are L ≈ 104 L ⊙ solar-mass objects that can exhibit large periods of extinction from dust ejection episodes. Many exhibit semi-regular pulsations in the range of 30–50 days with semi-amplitudes of 0.05–0.3 mag. Space-based photometry has discovered that solar-like oscillations are ubiquitous in hydrogen-dominated stars that have substantial outer convective envelopes, so we explore the hypothesis that the pulsations in RCB stars and the closely related dustless hydrogen-deficient carbon (dLHdC) stars, which have large convective outer envelopes of nearly pure helium, have a similar origin. Through stellar modeling and pulsation calculations, we find that the observed periods and amplitudes of these pulsations follows the well-measured phenomenology of their H-rich brethren. In particular, we show that the observed modes are likely of angular orders l = 0, 1, and 2 and predominantly of an acoustic nature (i.e., p-modes with low radial order). The modes with largest amplitude are near the acoustic cutoff frequency appropriately rescaled to the helium-dominated envelope, and the observed amplitudes are consistent with that seen in high-luminosity (L > 103 L ⊙) H-rich giants. We also find that for T eff ≳ 5400 K, an hydrogen-deficient carbon stellar model exhibits a radiative layer between two outer convective zones, creating a g-mode cavity that supports much longer period (≈100 days) oscillations. Our initial work was focused primarily on the adiabatic modes, but we expect that subsequent space-based observations of these targets (e.g., with TESS or Plato) are likely to lead to a larger set of detected frequencies that would allow for a deeper study of the interiors of these rare stars.

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Section: Introductionsupporting

confidence: 74%

The Asteroseismological Richness of RCB and dLHdC Stars

Wong,

Bildsten

2024

ApJ

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“…They have been found to be less luminous than their dusty counterparts by an average of ∼2 mag, indicating that they could originate from whitedwarf mergers of lower total mass compared to that of RCB stars. Furthermore, dLHdC stars present possibly an even lower O 16 /O 18 isotopic ratio (Karambelkar et al 2022) than the one measured in RCB stars (Clayton et al 2007;García-Hernández et al 2010). However, the boundary between RCB and dLHdC stars is not clear cut, as a few dLHdC stars (F75, F152, C526, and A166) have been found to produce a small amount of dust (a weak IR excess signal was detected) and even be in the same brightness regime as RCB stars.…”

Section: Introductionmentioning

confidence: 82%

The dawn of a new era for dustless HdC stars withGaiaeDR3

Tisserand

Crawford

Clayton

et al. 2022

A&A

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Context. Decades after their discovery, only four hydrogen-deficient carbon (HdC) stars were known to have no circumstellar dust shell. This is in complete contrast to the ∼130 known Galactic HdC stars that are notorious for being heavy dust producers, i.e. the R Coronae Borealis (RCB) stars. Together they form a rare class of supergiant stars that are thought to originate from the merger of CO/He white dwarf (WD) binary systems, otherwise known as the double degenerate scenario. Aims. We searched for new dustless HdC (dLHdC) stars to understand their Galactic distribution, to estimate their total number in the Milky Way and to study their evolutionary link with RCB stars and extreme Helium stars, the final phase of HdC stars. Methods. We used primarily the 2MASS and GAIA eDR3 all-sky catalogues to select candidates that were then followed up spectroscopically. We studied the distribution of known and newly discovered stars in the HR diagram. Results. We discovered 27 new dLHdC stars, one new RCB star, and two new EHe stars. Surprisingly, 20 of the new dLHdC stars share a characteristic of the known dLHdC star HD 148839, having lower atmospheric hydrogen deficiencies. The uncovered population of dLHdC stars exhibit a Bulge-like distribution, like the RCB stars, but show multiple differences from those that indicate they are a different population of HdC stars following its own evolutionary sequence with a fainter luminosity and also a narrow range of effective temperatures, between 5000 and 8000 K. Not all the new dLHdC stars belong to that new population, as we found indication of a current low dust production activity around four of them: the warm F75, F152, C526 and the cold A166. Those could in fact be typical RCB stars passing through a transition time, going in or out of the RCB phase. Conclusions. For the first time we have evidence of a large range of absolute magnitude in the overall population of HdC stars, spanning more than 3 mag. In the favoured formation framework, this is explained by a large range in the initial total WD binary mass which leads to a series of evolutionary sequences with distinct maximum brightness and initial temperature. The cold Galactic RCB stars are also noticeably fainter than the Magellanic ones, possibly due to a difference in metallicity between the original population of stars resulting in different WD mass ratio. The unveiled population of dLHdC stars indicates that the ability to create dust could be linked to the initial total mass. In our Galaxy, there could be as many dLHdC stars as RCB stars.

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“…Our 16 O/ 18 O = 4 for DY Per is within the range reported by the authors for their extensive sample of H-deficient giants. Karambelkar et al (2022) claim that the HdC and RCB stars have "different oxygen isotope ratios": six of the HdC stars have 16 O/ 18 O < 0.5 but in contrast 28 of the 33 RCB stars have 16 O/ 18 O > 1 but 12 RCB stars are determined to have 16 O/ 18 O > 50 and even eight of this sample have 16 O/ 18 O > 500. It is surely, as Karambelkar et al (2022) note, vital to confirm the distribution function for 16 O/ 18 O among HdC and RCB stars from high-resolution infrared spectra but these initial differences may point to a difference in the origins of HdC and RCB stars.…”

Section: Isotopic Oxygenmentioning

confidence: 96%

“…Karambelkar et al (2022) claim that the HdC and RCB stars have "different oxygen isotope ratios": six of the HdC stars have 16 O/ 18 O < 0.5 but in contrast 28 of the 33 RCB stars have 16 O/ 18 O > 1 but 12 RCB stars are determined to have 16 O/ 18 O > 50 and even eight of this sample have 16 O/ 18 O > 500. It is surely, as Karambelkar et al (2022) note, vital to confirm the distribution function for 16 O/ 18 O among HdC and RCB stars from high-resolution infrared spectra but these initial differences may point to a difference in the origins of HdC and RCB stars. Intriguingly, the extension of this hint to DY Per variables could yield insight into a possible relation between these variables and the HdC/RCB stars.…”

Section: Isotopic Oxygenmentioning

confidence: 96%

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The Carbon Star DY Persei May Be a Cool R Coronae Borealis Variable

García-Hernández

Rao

Lambert

et al. 2023

ApJ

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Optical and near-IR photometry suggests that the carbon star DY Persei exhibits fadings similar to those of R Coronae Borealis (RCB) variables. Photometric surveys of the Galaxy and Magellanic Clouds uncovered new DY Per variables with infrared photometry identifying them with cool carbon stars, perhaps, with an unusual tendency to shed mass. In an attempt to resolve DY Per’s identity crisis—a cool carbon giant or a cool RCB variable?—we analyze a high-resolution IGRINS H&K-band spectrum of DY Per. The CO first-overtone bands in the K band of DY Per show a high abundance of 18O such that 16O/18O = 4 ± 1, a ratio sharply at odds with published results for regular cool carbon giants with 16O/18O ∼ 1000 but this exceptionally low ratio is characteristic of RCB variables and HdC stars. This similarity suggests that DY Per indeed may be a cool RCB variable. Current opinion considers RCB variables to result from the merger of a He onto a CO white dwarf; observed abundances of these H-deficient stars including the exceptionally low 16O/18O ratios are in fair accord with predicted compositions for white dwarf merger products. An H-deficiency for DY Per is not directly observable but is suggested from the strength of an HF line and an assumption that F may be overabundant, as observed and predicted for RCB stars.

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R Coronae Borealis and dustless hydrogen-deficient carbon stars likely have different oxygen isotope ratios

Cited by 11 publications

References 41 publications

The Asteroseismological Richness of RCB and dLHdC Stars

The Asteroseismological Richness of RCB and dLHdC Stars

The dawn of a new era for dustless HdC stars withGaiaeDR3

The Carbon Star DY Persei May Be a Cool R Coronae Borealis Variable

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